1. Field of the Invention
The present invention relates to a photocurrent sensing circuit and, more particularly, to a photocurrent sensing circuit having a stabilized feedback loop that converts a light signal into an electrical signal.
2. Description of the Related Art
A circuit presented in U.S. Pat. No. 6,104,020 is illustrated in FIG. 1 as the prior art.
Referring to FIG. 1, the conventional photocurrent sensing circuit includes a current source 1 that applies a driving current of a photocurrent sensing circuit; a photocurrent generating unit that generates and amplifies a photocurrent in response to the amount of light incident into the photocurrent sensing circuit; a feedback loop that stabilizes an operating voltage of the photocurrent generating unit 2; a voltage signal output unit 12 that outputs a voltage signal proportional to the photocurrent generated by the photocurrent generating unit 2; and a switching unit 9 that steers the photocurrent of the photocurrent generating unit 2 toward the voltage signal output unit 12 or power voltage Vdd depending on a status of an output control signal 13. The photocurrent generating unit 2 includes a photodiode 3 that generates the photocurrent in response to the amount of light incident into the photocurrent sensing circuit; a parasitic capacitor 4 of the photodiode 3; and a PNP transistor 5 that amplifies the photocurrent generated by the photodiode 3.
The feedback loop 6 includes an NMOS transistor 7 where a drain node is connected to an NMOS transistor 10 of the switching unit 9, a source node is connected to the PNP transistor 5 of the photocurrent generating unit 2, and a gate node is connected to the current source 1 to provide a feedback loop generation current in response to operation of the NMOS transistor 10 of the switching unit 9; an NMOS transistor 8 where a drain node is connected to the current source 1, a gate node is connected to a cathode node of the photodiode, and a source node is connected to an anode node 3 of the photodiode of the photocurrent generating unit 2 to provide the feedback loop generation current in response to the current source 1; and the PNP transistor 5 of the photocurrent generating unit 2.
The switching unit 9 includes an NMOS transistor 10 where a gate node is connected to an output control terminal 13, a drain node is connected to a power voltage Vdd, and a source node is connected to the feedback loop 6 to control a current supply of the drain node of the NMOS transistor 7 in response to an output signal of the output control terminal 13; and an PMOS transistor 11 where a gate node is connected to the output control terminal 13, a drain node is connected to a storage capacitor 12, and a source node is connected to the feedback loop 6 to switch the photocurrent delivered through the NMOS transistor 7 into the voltage signal output unit 12 in response to the output signal of the output control terminal 13.
The voltage signal output unit 12 includes the storage capacitor 12 that generates a voltage signal proportional to the photocurrent generated by the photocurrent generating unit 2.
Operation of the photocurrent sensing circuit of FIG. 1 described above is as follows.
When the light is illuminated into the photodiode 3, a photocurrent is generated proportional to the amount of light. The photocurrent flows as a base current of the PNP transistor 5 as long as a voltage of a base node of the PNP transistor 5 remains constant through the feedback loop 6. As such, when the photocurrent flows as the base current of the PNP transistor 5, a current amplified by a current amplification factor, i.e., a gain of the PNP transistor 5 flows in the NMOS transistor 7.
The current delivered through the NMOS transistor 7 is switched depending on the status of the output control signal of the output control terminal 13 to be output as a voltage signal of the storage capacitor 12 or driven into the feedback loop circuit to keep constant the base voltage of the PNP transistor 5. First, when an output control signal having a Shutter On value is applied to the switching unit 9 from the output control terminal 13, the PMOS transistor 11 of the switching unit 9 turns on and the NMOS transistor 10 of the switching unit 9 turns off in response to the output control signal.
Therefore, the current delivered through the NMOS transistor 7 allows charges stored in the storage capacitor 12 of the voltage signal output unit 12 to be discharged.
Thus, a current proportional to the discharged amount of charges is generated in the voltage signal output unit 12, and a voltage signal is generated proportional to the generated current. Further, the generated voltage signal is outputted via the output terminal 14.
On the contrary, when an output control signal having a Shutter Off value is applied from the output control terminal 13 to the switching unit 9, in the switching unit 9, the NMOS transistor 10 turns on and the PMOS transistor 11 turns off in response to the output control signal, i.e. the Shutter Off.
When the storage capacitor 12 does not discharge the charges, the current delivered through the NMOS transistor 10 serves to provide a current path that keeps stable the feedback loop having the NMOS transistors 7, 8 and the PNP transistor 5.
A good photocurrent sensing circuit requires high efficiency in converting light into a voltage, a wide operating range in both dark and bright environment, a high signal-to-noise ratio (SNR) and fast response characteristics.
In the prior art shown of FIG. 1, the feedback loop formed by coupling between the NMOS transistors 7, 8 and the PNP transistor 5 forms a stable circuit, thereby having high efficiency, a wide operating range and a high SNR.
However, much attention should be paid to keeping the feedback loop stable. This is because the NMOS transistor 7 and the PMOS transistor 11 may simultaneously turn off to make the feedback loop temporarily unstable if the timing is not maintained in driving the output control signal as an ideal pulse signal of the output control terminal 13. During transition time of the output control terminal 13, both NMOS transistor 10 and PMOS transistor 11 turn off. Further, when the overall photocurrent sensing circuit is driven using the output control signal, if the amount of light incident into a specific photocurrent sensing circuit is large, the amplitude of the photocurrent generated through the photocurrent generating unit 2 of the photocurrent sensing circuit becomes larger proportional to the amount of incident light, and accordingly the voltage of the storage capacitor 12 is rapidly lowered.
In this case, the voltage of an emitter node of the PNP transistor 5 is lowered, so that the PNP transistor 5 turns off and the feedback loop is not maintained normally, causing each node voltage of the feedback loop to be unstable.
Such an unstable feedback loop also leads to the unstable voltage of the base node of the PNP transistor 5, which degrades the overall photocurrent conversion characteristics of the photocurrent sensing circuit.
Further, while the PNP transistor 5 has an advantage that a current generated by light is amplified, the current amplification factor of the PNP transistor 5 is not uniform among pixels due to a process variation, so that it exerts a bad influence on the overall uniformity of the photocurrent sensing circuit.